The present invention pertains to the aircraft flight control art and, more particularly, to an improved aircraft autopilot flight director system.
Several aircraft autopilot flight director systems are well known in the prior art. Basically, the function of an autopilot flight director system is to process a series of input signals, such as aircraft acceleration, altitude, gyro data, pilot commands and guidance commands and produce an output command signal suitable for driving an aircraft control surface to control flight in a predetermined manner. Inasmuch as the autopilot flight director system can be in control of aircraft flight during safety critical portions of the flight envelope, such systems normally include redundancy with failure detection to assure continued safe flight following failures within the system.
One such prior art system employs dual/dual control channel redundancy with each control channel including two parallel sensor and computational channels. Here, the two independent, parallel control channels process input data to produce output control signals. Each control channel has redundant sensor inputs which are not shared by the other control channel. Flight computers within each control channel process the redundant sensor information to produce an output command signal which is torque summed with the command signal from the parallel control channel via servo motor actuators and a combining shaft. The shaft is coupled to an aircraft control surface. Each flight computer generates not only an output command signal but also a servo model which provides a simulated channel output. Position feedback sensors provide to each flight computer the status of its corresponding servo motors, whereby the servo outputs may be compared with the servo model. If a significant discrepancy exists between the actual servo output and that of the model, a failure is indicated and the output servo is isolated from the system.
In existing triplex systems, three independent channels, each with its own sensor input data, operate in parallel. The command signal outputs from each channel are coupled through servo motor actuators to a common shaft and control surface.
Both the dual/dual and triplex systems are capable of satisfactory operation in that both provide fail-operational performance following any first channel failure and are capable of providing fail-passive performance following the combination of two channel failures. However, both systems employ a substantial amount of hardware. The dual/dual system normally requires at least four redundant sensor sets, while the triplex system requires only three sensor sets. This additional hardware adds significantly to the cost of the systems. Moreover, in applications such as aircraft wherein weight savings are crucial, elimination of hardware without loss of system performance is a constant goal.